In NAND technology, programming is performed in parallel simultaneously on all cells along a word line (WL) that are to be programmed. The time taken to complete a programming operation is referred to as Tprog, and is an important figure of merit for a NAND device. As NAND devices scale smaller, the effects of parasitic coupling capacitance continue to increase and cause a longer time to charge word lines to high voltages.
Due to several sources of variation, such as doping concentration, tunneling, structural, etc., there is usually a significant variation in the programming speeds of the different cells along a word line. The variation in programming speeds is a metric referred to as the Program Vt Sigma (PVS). A goal of a NAND programming technique is to obtain the tightest possible threshold voltage (Vt) distribution of NAND cells above a program verify (PV) Vt level for a given Tprog budget. In a conventionally used programming technique, the programming voltage (Vpgm) for the first programming pulse (Vpgm—min) of a programming operation is selected so that the fastest-to-program cells (“fast cells”) do not over-program. The final Vpgm (Vpgm—max) is determined by the slowest-to-program cells (“slow cells”), which need to be programmed up to the verify voltage. During the programming operation, Vpgm is stepped in increments (Gate_Step) from Vpgm—min to Vpgm—max to obtain the tightest possible threshold voltage (Vt) distribution above a program verify (PV) Vt voltage for a given Tprog budget.
It will be appreciated that for simplicity and/or clarity of illustration, elements depicted in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. The scaling of the figures does not represent precise dimensions and/or dimensional ratios of the various elements depicted herein. Further, if considered appropriate, reference numerals have been repeated among the figures to indicate corresponding and/or analogous elements.